The present invention relates to a liquid crystal display device having a liquid crystal display panel composed of a liquid crystal cell which seals a liquid crystal layer between a pair of transparent substrates giving optical change to incident light by applying voltage to the liquid crystal layer and polarizers provided in a visible side of the liquid crystal cell and the opposite side thereof respectively, and an auxiliary light source provided in an opposite side with respect to the visible side. Namely, it relates to a transmissive liquid crystal display device and a transflective liquid crystal display device.
Currently, among the liquid crystal display devices, there exist a transmissive liquid crystal display device which performs a display by lighting at an auxiliary light source provided therein all times, a reflective liquid crystal display device which performs a display utilizing light from an external light source (sunlight or illumination light source), and a transflective liquid crystal display device which performs a display utilizing reflecting light obtained from the external light source when the external light is bright, and performs the display utilizing transmission light obtained from a lighted inner auxiliary light source when the external light is dark.
The reflective liquid crystal display device is most effective to make the best use of the merit of low power consumption and thinness of a liquid crystal display device. However, since it can not perform display when external surroundings are dark, a transflective liquid crystal display device is widely used, which can perform display even if the external surroundings are dark by providing an auxiliary light source inside thereof.
An example of structure and function of a conventional transflective liquid crystal display device will be explained with FIG. 17 and FIG. 18. FIG. 17 is a plan view of a conventional transflective liquid crystal display device, and FIG. 18 is a schematic enlarged sectional view taken along the line 18xe2x80x9418 in FIG. 17. In FIG. 17, since most constituting members are transparent and a lower side member among members overlapping vertically, each other can be seen through, such a lower side member is also shown with solid lines. In the following explanation, a visible side which can be recognized by observers is taken as an upper side
In the above transflective liquid crystal display device, a liquid crystal cell (optical adjuster) 1 is composed such that a first substrate 2 and a second substrate 5 both made of a transparent material such as glass are opposedly disposed with a predetermined space and coupled to each other with a sealing member 16 to seal a liquid crystal layer 15 in-between thereof so that an optical change can be given to incident light by applying voltage on the liquid crystal layer 15.
The transflective liquid crystal display device comprises a first polarizer (polarized light separator) 21 provided on the outside (the upper side in FIG. 18) of the first substrate 2 which is in the visible side of the liquid crystal cell 1, and a second polarizer (polarized light separator) 22 provided on the outside (the lower side in FIG. 18) of the second substrate 5 which is on the opposite side with respect to the visible side, respectively, and an auxiliary light source 31 is provided on the opposite side of the second polarizer 22 with respect to the liquid crystal cell 1.
Further explaining the liquid crystal cell 1 in detail, as shown in FIG. 17, on the inner surface of the first substrate 2, stripe-shaped signal electrodes 3 made of an indium tin oxide (ITO) film which is a transparent conductive film are formed in a lateral direction with a predetermined space.
On the inner surface of the second substrate 5, a color filter 9 composed of red (R) color filters 6, green (G) color filters 7 and blue (B) color filters (not shown) is provided first, and a protective insulating film 10 is disposed thereon, then stripe-shaped scanning electrodes 11 made of ITO film are formed in a longitudinal direction with a predetermined space on the protective insulating film 10, as shown in FIG. 17.
The color filter 9 is formed in a state that the adjacent color filters having a color different from each other are overlapped slightly or abutted on each other. The protective insulating film 10 is provided to flatten the difference in level of the color filter 9 and to prevent deterioration of the signal electrodes 3 caused during the pattern forming process.
As shown in FIG. 17, an intersection point of the signal electrode 3 and the scanning electrode 11 intersecting to each other at right angles serves as a pixel portion 14, and a display region is formed with a number of the pixel portions 14 provided in a matrix.
Alignment layers (not shown) as treatment layers to align the liquid crystal layer 15 in a predetermined direction are provided respectively on the opposing surfaces of the first substrate 2 and the second substrate 5.
The first substrate 2 and the second substrate 5 are coupled with a fixed space therebetween with the sealing member 16 in a manner that the signal electrodes 3 on the first substrate 2 face the scanning electrodes 11 on the second substrate 5, and the liquid crystal layer 15 is filled through the opening provided in the sealing member 16, and then hermetically closed with a sealant 17 shown in FIG. 17.
Though a twisted nematic (TN) liquid crystal, a super twisted nematic (STN) liquid crystal, or a homeotropic liquid crystal are used for the liquid crystal layer 15, a twisted nematic (TN) liquid crystal having a twist angle of 90xc2x0 is assumed to be used in this explanation.
A first polarizer 21 provided on the upper side of the first substrate 2 in the liquid crystal cell 1 is an absorption-type polarizer (absorptive polarized light separator) in which one polarization axis is a transmission axis and another polarization axis intersecting with the transmission axis at nearly right angles is an absorption axis, and a second polarizer 22 provided on the bottom side of the second substrate 5 is a reflection-type polarizer (reflective polarized light separator) in which one polarization axis is a transmission axis and another polarization axis intersecting with the transmission axis at nearly right angles is a reflection axis. The first polarizer 21 and the second polarizer 22 are disposed in such a manner that both transmission axes intersect with each other at right angles.
A scattering layer 4 made of a mixture of transparent resin and polyvinyl resin beads having different refractive indices is provided between the second polarizer 22 and the second substrate 5, and a printed layer 23 made of translucent ink is further provided on the bottom surface of the second polarizer 22.
According to a ratio of absorption to transmission of the printed layer 23, brightness in a dark display state at the time of reflective display and in a bright display state at the time of transmissive display is determined.
The auxiliary light source 31 is disposed in further lower side of the second polarizer 22, which is composed of a fluorescent tube 32, a reflector 34, a diffuser 33 and a prism sheet (not shown) in this example.
In the transflective liquid crystal display device thus structured, as shown in FIG. 18, when an external light from a main light source, for instance sunlight, passes through the first polarizer 21 and the first substrate 2 of the liquid crystal cell 1, as a first incident light L1, and when made incident on a portion of the liquid crystal layer 15 in which no voltage is applied (background portion), the first incident light L1 is made incident on a color filter 9 while being optically rotated or being given a phase difference by the liquid crystal layer 15, passes through the second substrate 5 and the scattering layer 4 and comes to the second polarizer.
The first incident light L1 passes through the above described light course, which causes absorption by respective members, light attenuation arises when it becomes an incident light into the second polarizer 22.
A linearly polarized light component of the first incident light L1 passed through the first polarizer 21 is optically rotated at an angle of 90xc2x0 by the liquid crystal layer 15, made incident on the second polarizer 22 in a direction parallel to the transmission axis thereof and passes therethrough. However, absorption occurs due to the printed layer 23 disposed on the bottom surface of the second polarizer 22, so that a first reflected light L3 reflected by the printed layer 23 becomes very weak in intensity and hardly emitted to the observer""s side.
A second incident light L2 from the external light source also reaches the second polarizer 22 after passing through the same course as in the case of the first incident light L1, it passes through a portion in which voltage is applied on the liquid crystal layer 15 (display portion) by the signal electrodes 3 and the scanning electrodes 15 in the liquid crystal cell 1 into the second polarizer 22.
In this case, a linearly polarized light component of the second incident light L2 passed through the first polarizer 21 is made incident on the second polarizer 22 without being optically rotated at an angle of 90xc2x0 when it passes through the liquid crystal cell 15. Accordingly, since the incident light becomes a light linearly polarized in a direction parallel to the reflection axis of the second polarizer which is a reflection-type polarizer, all the incident light are reflected by the second polarizer 22. Then scattering property is given by the scattering layer 4 and it becomes a second reflection light L5, a third reflection light L6, a fourth reflection light L7 and so on which respectively pass through the color filters 9 to be emitted into the observer""s side.
Therefore, a color display in bright can be visually recognized in a dark background.
When a fluorescent tube 32 of an auxiliary light source 31 is on, the emitted light L4 from the auxiliary light source 31 are partially absorbed by the printed layer 23, a linearly polarized light component polarized in a direction parallel to the transmission axis of the second polarizer 22 passes through the second polarizer 22, then passes through the color filters 9 of the liquid crystal cell 1 after scattering property being given by the scattering layer 4, and is made incident on the liquid crystal layer 15.
When the incident light passes through a portion of the liquid crystal layer 15 in which no voltage is applied (background portion), since the linearly polarized light is optically rotated at an angle of 90xc2x0, and made incident on the first polarizer 21 as a light linearly polarized in a direction parallel to the transmission axis thereof, the incident light is emitted into the observer""s side after passing through the first polarizer 21. However, when the incident light passes through a portion of the liquid crystal layer 15 in which voltage is applied (display portion), the linearly polarized light is not optically rotated at an angle of 90xc2x0 and is made incident on the first polarizer 21 which is an absorption-type polarizer, as a light linearly polarized in a direction parallel to the absorption axis thereof, thereby the incident light being absorbed by the first polarizer 21, and hardly emitted into the observer""s side.
Therefore, a display in a dark color (black) can be visually recognized in a bright background.
In the transflective liquid crystal display device described above, in a case of a display using an external light source, the use of a reflection-type polarizer as the second polarizer 22 has the following advantages compared with a case of using a combination of an absorption-type polarizer and a transflective reflector.
When an absorption-type polarizer is used, since a bright display utilizes light reflected by a transflective reflector disposed in the bottom side of the absorption-type polarizer, the light passes through the absorption-type polarizer twice, which increases attenuation due to the absorption of the light.
Furthermore, the reflection intensity is lowered because the transflective reflector has a transmission property. In other words, the more the reflection property is enhanced, the lower the transmission property becomes, or the more the transmission property is enhanced, the lower the reflection property becomes. Because of these, the quantity of light emitted into the observer""s side is decreased, and brightness of the display is lowered.
When a reflection-type polarizer is used, on the contrary, since a bright display is achieved by reflection in a reflection axis of the reflection-type polarizer and scattering property of the scattering layer 4 provided between the reflection-type polarizer and the second substrate 5, light attenuation can be reduced. The transmission property is also maintained by means of the polarization axis (transmission axis). Therefore a bright display in a light state can be realized.
A case of a dark display will be explained next. In a case of using an absorption-type polarizer and a transflective reflector as the second polarizer 22, when a linearly polarized light component polarized in a direction parallel to the absorption axis is made incident on the absorption-type polarizer, the light is absorbed and does not reach the transflective reflector, which makes the display dark, because the reflection property of the absorption-type polarizer is weak.
In a case of using a reflection-type polarizer, when a light linearly polarized in a direction parallel to the transmission axis is made incident on the reflection-type polarizer, the light passes through the reflection-type polarizer and is absorbed by the printed layer 23, which makes the light reflected into the observer""s side from the reflection-type polarizer weak, thereby obtaining a dark display.
Thus, when a transflective liquid crystal display device is used as a reflective liquid crystal display device utilizing external light source, the contrast ratio is about 10 to 1 due to the adoption of design which regards brightness as important because the visibility is lowered in a dark display, and due to expectation of reflection of the external light and reflecting-in of the outside surroundings on the surface of the liquid crystal display panel. Therefore it is possible to use satisfactorily even the polarization degree of the second polarizer is low (for instance, even as low as a level of 92%).
In addition, even when the polarization degree of the reflection-type polarizer, especially the polarization degree of the reflection axis, is partially changed, it presents no problem because of lowness of the contrast and the brightness, and weakness in observer""s sensitivity to the difference in brightness because of using it for a bright display.
Furthermore, if the polarization degree of the reflection axis of the reflection-type polarizer has a wavelength dependency in the visible light region, and the reflection-type polarizer has an insufficient portion in polarization degree, it presents little problem because of the weakness in sensitivity of the observer.
However, when the transflective liquid crystal display device is used as a transmissive liquid crystal display device by lighting the auxiliary light source 31 disposed on the bottom side of the second polarizer 22 shown in FIG. 18, since the contrast ratio is increased, though depending on the intensity of the light source, and since the reflection axis of the reflection-type polarizer serves to block the light, it becomes a dark display.
Accordingly, only a slight difference in contrast, or a slight difference in color can be recognized by observers. Additionally, in a case of the transflective liquid crystal display, since an auxiliary light source is used in a dark surroundings, the observer""s sensitivity increases against unevenness of the dark display and coloring thereof, so that a slight unevenness in display results in the recognition by observers.
In a case of a transflective liquid crystal display device having color filters, color filters having high transmittance are used to obtain a display as bright as possible when used as a reflective one.
As a result, even when an auxiliary light source having a weak light emitting intensity compared with that in the transmissive liquid crystal display device is used to reduce power consumption, it becomes a comparatively bright display. Consequently, positional distribution of the polarization degree of the second polarizer 22 can be recognized by the observers as the distribution of the contrast ratio.
Further, the difference in the polarization degree depending on wavelength in the visible light region causes coloring of the dark display.
As a countermeasure against this, the difference in the contrast ratio and difference in color can be reduced to the extent almost not recognizable by arranging the polarization degree of the second polarizer 22 to be about 99.9% so that the difference in the polarization degree is about 1%, but there is a limit in the polarization degree so far as using a thin and inexpensive reflection-type polarizer.
The present invention is achieved in view of the above-described disadvantages when using a conventional transflective liquid crystal display device as a transmissive liquid crystal display device by lighting an auxiliary light source, and the object thereof is to provide a thin and inexpensive liquid crystal display device without impairment of portability, in which a display quality in a reflection display and transmission display of a transflective liquid crystal display device is improved and in particular, reduction of unevenness of the display and unevenness in coloring in a case of the transmission display can be realized.
The present invention relates to a liquid crystal display device comprising a liquid crystal cell formed by opposedly disposing a transparent first substrate and a transparent second substrate with a predetermined space, filling a liquid crystal layer in-between thereof so that an optical change is given to incident light by applying voltage on the liquid crystal layer, a first polarizer provided on the outside of the first substrate which is in the visible side of the liquid crystal cell, a second polarizer provided on the outside of the second substrate which is in the opposite side to the visible side of the liquid crystal cell, and an auxiliary light source provided on the opposite side of the second polarizer with respect to the liquid crystal cell. In order to achieve the objects described above, a third polarizer is provided between the second polarizer and the auxiliary light source to enhance the polarization degree when the light from the auxiliary light source is emitted into the liquid crystal cell side as a polarized light after passing through the second polarizer.
The first polarizer is an absorption-type polarizer in which one polarization axis is a transmission axis and another polarization axis intersecting to the transmission axis at nearly right angles is an absorption axis, and the second polarizer is a reflection-type polarizer in which one polarization axis is a transmission axis and another polarization axis intersecting to the transmission axis at nearly right angles is a reflection axis.
A reflection-type polarizer made of a multilayer composed of a plurality of complex layers formed by combining a layer having nearly equal refractive indices in a predetermined direction and in a direction orthogonal thereto and a layer having different refractive indices in a predetermined direction and in the direction orthogonal thereto is used as the reflection-type polarizer.
The third polarizer is an absorption-type polarizer or a reflection-type polarizer.
Further, the first polarizer and the second polarizer are disposed such that the transmission axes thereof intersect at right angles, and the second polarizer and the third polarizer are disposed such that the transmission axes thereof form an angle from 0xc2x0 to plus or minus 30xc2x0 (especially, from 0xc2x0 to plus or minus 10xc2x0 is preferable).
The third polarizer is preferably composed such that a plurality of either reflection-type polarizer or absorption-type polarizer, or both of them, are piled up.
In such a case, transmission axes of respective polarizers composing the third polarizer are preferably arranged at different directions from each other.
Color filters in plural colors different in transmission intensity in the visible light region may be disposed between the first polarizer and the second polarizer, the auxiliary light source may be made to be a light source emitting light containing transmission wavelengths of the color filters in plural colors, and light emitted from the auxiliary light source may be transmitted through the third polarizer and the second polarizer and transferred to the color filter as a linearly polarized light having little in-plane distribution of polarization degree and little difference of polarization degree in the visible light region.
A scattering layer to scatter light is preferably provided at least on a position in the visible side of the first polarizer, between the first polarizer and the first substrate, or between the first substrate and the second substrate, and the scattering layer is preferably a layer transmitting light with little phase difference between the incident light side and the emitted light side, and without changing the polarization degree.
A light attenuation layer which transmits a part of light and absorbs or diffuses the other light is preferably disposed between the second polarizer and the third polarizer or between the third polarizer and the auxiliary light source.
Further, a retardation film may be provided between the second substrate and the second polarizer or between the second polarizer and the third polarizer.
Furthermore, a minute unevenness (a rough surface) is preferably formed on opposing surfaces of the third polarizer and the auxiliary light source, respectively.
The second polarizer and the third polarizer may be fixed each other with an adhesive layer.
A space is preferably provided between the third polarizer and the auxiliary light source.
A light attenuation layer to increase light absorption or light scattering is preferably provided on a surface of the auxiliary light source facing the third polarizer.
In a case where the third polarizer is a polarizer composed such that a plurality of either reflection-type polarizer or absorption-type polarizer, or both of them, are piled up, either a printed layer which transmits a part of light and absorbs the other light or a space, or both of them, are preferably provided between the plurality of polarizers composing the third polarizer.
In a case where both of the second polarizer and the third polarizer are reflection-type polarizers, the printed layer is preferably provided on a surface of the second polarizer opposing the third polarizer, and a space is preferably provided between the printed layer and the third polarizer. Further, a retardation film is preferably provided between the printed layer and the third polarizer.
The printed layer preferably has no polarization activity and no retardation, and one containing white fluorescent pigment may be used.
The liquid crystal display device according to the present invention (transflective liquid crystal display device) can correct the insufficiency in polarization degree and in-plane unevenness in polarization degree in the visible light region within the second polarizer by the third polarizer, and thereby reduces unevenness in display and unevenness in coloring in the transmission display using an auxiliary light source without increasing the polarization degree of the second polarizer itself, by providing the first polarizer which is an absorption-type polarizer on the visible of the liquid crystal cell and the second polarizer which is a reflection type polarizer on the opposite side of the visible side of the liquid crystal cell such that the transmission axes thereof intersect at right angles, and by further providing the third polarizer between the second polarizer and the auxiliary light source such that the transmission axis thereof is nearly coincide with that of the second polarizer.
In a case of a reflective display, since no absorption occurs in a polarizer compared with the conventional case of using an absorption-type polarizer because of utilizing reflection from the reflection axis of the second polarizer which is a reflection-type polarizer, a display in bright can be realized.
Especially, by using a third polarizer having polarization degree higher than that of a reflection-type polarizer used as the second polarizer, or having a smaller distribution in polarization degree in the visible light region, the polarization degree of light emitted from the second polarizer has no unevenness in the visible light region so that the evenness of display quality of the liquid crystal display device can be extensively improved.
In a case of a liquid crystal display device provided with a color filter, since light emitted from an auxiliary light source contains transmission wavelength of the color filter, wavelength dependency of the second polarizer becomes more remarkable. However, since the wavelength dependency can be reduced by the third polarizer, the evenness of display quality can be extensively improved.
Furthermore, since the polarization degree of the second polarizer can be improved by combining the second polarizer with the third polarizer, the contrast ratio of the display can be improved.
When the liquid crystal display device is used as a reflective display device utilizing an external light source (main light source) with the auxiliary light source being off, since a dark display is performed by utilizing light incident on the transmission axis of the second polarizer, it is necessary to prevent light from reflecting into the observer""s side caused by entering the light into the second polarizer again from the bottom side of the second polarizer.
Accordingly, it becomes possible to improve the quality and increase the contrast ratio in a dark display when using as a reflective display by providing a light attenuation layer to reduce reflection of light from the external light source into the second polarizer by transmitting the light partially and absorbing or scattering the rest of the light, between the second polarizer and the third polarizer, or between the third polarizer and the auxiliary light source.
Besides, since there is no need of considering deterioration of the polarization degree due to the light attenuation layer in a case the light attenuation layer being provided between the third polarizer and the auxiliary light source, selection of material for the light attenuation layer is facilitated and further in a case of a transmissive display, lowering in the polarization degree of the third polarizer and the second polarizer can be prevented. A printed layer is an example of the light attenuation layer.
In addition, by using an absorption-type polarizer or a reflection-type polarizer as the third polarizer, it becomes possible to improve the polarization degree of the second polarizer easily in a thin-type device. Especially, in order to improve the polarization degree and to reduce cost, it is efficient to utilize an absorption-type polarizer.
In a case of using a light attenuation layer, it becomes a dark display due to light absorption or scattering in the light attenuation layer. Further, since it transmits a portion of light, in-plane unevenness in transmittance is apt to arise, which makes it difficult to ensure the evenness of the display.
Accordingly, it is necessary to have an offset angle between the transmission axis of the second polarizer and the transmission axis of the third polarizer in a range of 0xc2x0 to plus or minus 30xc2x0.
The polarization degree of light from the auxiliary light source after passing through the third polarizer and the second polarizer becomes maximum at zero degree of the offset angle and declines gradually in accordance with the increase in absolute value of the offset angle. However, when the offset angle is in a range of plus or minus 30xc2x0, the polarization degree can be improved more sufficiently compared with a case when the second polarizer is used alone.
When it is used as a reflective display, by allowing the transmission axis of the second polarizer and the third polarizer to have an offset angle, it becomes possible to reduce the quantity of light incident on the third polarizer after passing through the second polarizer.
Further, the quantity of transflective light obtained through the transmission axis of the second polarizer and the transmission axis of the third polarizer can be very even in a plane, which improves the evenness of display compared with a case when a new light attenuation layer is used.
As described above, the offset angle of the transmission axis is preferably in a range of 0xc2x0 to plus or minus 30xc2x0, but in particular, in order to prevent lowering of the quantity of light emitted from the auxiliary light source, the range from 0xc2x0 to plus or minus 10xc2x0 is most suitable.
In addition, by providing plural layers of the third polarizer, improvement in the polarization degree when a light from the auxiliary light source passes through the second polarizer, and in the contrast ratio when it is used as a reflective display is realized.
In such a case, it is preferable to dispose the transmission axis of the third polarizer in the second polarizer side in nearly parallel to the transmission axis of the second polarizer, and to provide an offset angle between the transmission axis of the third polarizer in the auxiliary light source side and the transmission axis of the second polarizer.
That is, by providing a plurality of the third polarizers and the offset angles between respective transmission axes thereof, ensuring of the contrast ratio of the reflective display utilizing external light and improvement in the evenness in display of the transmissive display utilizing an auxiliary light source can be achieved.
In addition, by providing a space between the third polarizer and the auxiliary light source, reflection of light from the auxiliary light source can be reduced in a case of the reflection-type display. In particular, by providing an uneven surface on the bottom surface of the third polarizer, since a light is made incident on the third polarizer, canceling the reflection of the incident light from the auxiliary light source, absorption occurs, and thereby reducing the reflection intensity, which makes the dark display desirable.
Further, by bonding the third polarizer and the second polarizer together, or the third polarizer and the auxiliary light source together, change of the transmission axis of the second polarizer and the transmission axis of the third polarizer can be avoided when the liquid crystal display device is in use.